This invention relates to a method and apparatus for changing dies on a moving bolster for a press, particularly a large press. More particularly, this invention relates to a hydraulic control circuit for the moving bolster to decelerate the moving bolster to a smooth stop within the press. Still more particularly, this invention relates to a rotary valve for use in a circuit of the type described for controlling the movement of a moving bolster during die changing.
Large presses are well known for making large items such as automobile body parts by pressing a metal blank. Typically, such presses include a press body and a pressing apparatus for pressing a metallic blank inserted intermediate the pressing apparatus and a die positioned in a bolster. It is a continuing problem in this art to provide a means for changing dies on a bolster to facilitate die changes within the press.
A typical arrangement utilizes a moving bolster. For a die change, the bolster containing the existing die, for forming automobile side panels for example, is withdrawn from the press. Then, the automobile side panel die is removed from the bolster and another die, such as a hood die, is secured to the bolster. Thereafter, the bolster with the hood die attached is moved into position within the press and secured at that point. The art of presses, moving bolsters, and securing dies to the moving bolster is well-developed.
However, it remains a problem in this art to expedite die changes. In addition to minimizing production interruption for a die change in normal circumstances, it is desired to minimize inventory of a particular part for cost reasons. Thus, while large numbers of a particular part might be pressed before changing to another part because of the long time needed to change the dies, now it is a significant desire in the art to produce fewer parts before a die change. This requires therefore that die changes be expedited and completed in a short time.
Such presses are massive with capacities up to hundreds of tons of pressure to be exerted on dies which can weigh in the order of 40 or 50 tons. Accordingly, the bolsters and the dies are able to withstand such pressures in the pressing operation, largely through their size and structure. It is a problem, however, when undergoing expedited die changing of the type described, to move such massive parts in a way which satisfactorily controls movement and momentum of the combination, both in withdrawing the die/bolster combination from the press, but particularly in inserting the new die/bolster combination into the press. It is of course desirable to move the die and bolster as promptly as possible, but a risk of damage to the equipment for failure to stop adequately must be avoided given the masses involved.
Accordingly, it is a problem in this art to provide a control circuit, such as pneumatic control circuit compatible with typical control circuits for moving such equipment, which can control acceleration, deceleration, and stopping of such equipment.
It is another problem in this art to automate such stopping particularly within the press, since operator judgment if faulty can cause significant damage for failure of the equipment to stop precisely within the press. Thus, it is desired to provide a circuit which is responsive to a ramp on the track of travel to initiate deceleration and stoppage.
It is another problem in this art to provide a control valve responsive to the ramp for use in the hydraulic circuit of the invention to control deceleration and stoppage of the die and bolster at a predetermined time, with a desired hold and subsequent release of the pneumatic braking.
These and other objects of the invention will be apparent from a detailed description of the invention which follows.